The system monitor and calculator apps are installed with snap in Ubuntu 18.04 and from time to time you might find you won't be able to open these apps. To resolve this simple uninstall both apps and reinstall with the following commands:

snap remove gnome-calculator

snap remove gnome-system-monitor

snap install gnome-calculator

snap install gnome-system-monitor

If this doesn't sort out the issue, you can remove the both apps with snap as shown above and then install the deb packages as follows:

sudo apt install gnome-system-monitor

sudo apt install gnome-calculator